(a) Field of the Invention
This invention relates to a process for producing ceramic oxide fibers from a solvent solution of an oxide precursor which is soluble in the solvent. More particularly, the invention relates to such a process wherein the resulting fibers have very small diameters.
(b) History of the Prior Art
It is known in the prior art that certain oxide precursors may be dissolved in liquid and then spun in the form of fiber and the resulting fiber heated in an oxygen containing atmosphere to form an oxide fiber. For example, certain solvent soluble salts of metals whose oxides are ceramics can be used. Examples of such inorganic salts are aluminum chloride, basic aluminum chloride and zirconium oxychloride. Examples of suitable organic salts are aluminum, chromium, zirconium or titanium salts of a lower alkyl organic acid such as aluminum acetate, aluminum chloroacetate, aluminum formate, chromium oxyacetate, zirconium acetate and titanium acetate. In addition, some salts can be utilized in blends or mixes. It is to be understood that essentially any compound, which can be dissolved in a solvent to form a solution which can be fiberized and which results in a ceramic oxide fiber upon heating to a sufficient temperature, can be used as a ceramic oxide precursor. For example, basic aluminum chloride, which is an especially desirable ceramic oxide precursor, may be dissolved in water and the resulting solution spun to form a basic aluminum chloride fiber which upon heating, converts to a polycrystalline aluminum oxide fiber. Colloidal silica may also be included in the solution.
Difficulties were encountered in prior art processes for the manufacture of such fibers from solutions and in particular, it was difficult to form fibers having very small diameters, i.e., less than 5 and preferably less than 4 microns.
It has been known that certain molten materials can be fiberized by placing heat softened or melted material into a rotatable hollow element having aperatures in an outer peripheral wall followed by rotating the element to force the molten material through the aperatures to form fibers. It has also been known that such fibers made from molten material, while still soft immediately after leaving the element, can be contacted with a stream of gaseous fluid to assist in forming fine fibers (see e.g., U.S. Pat. No. 3,177,058 to G. Slayter et al and U.S. Pat. No. 4,111,673 to Van Natta).
It was not, however, recognized that such an apparatus would have any utility in forming fibers having diameters below 5 microns when the fibers are formed from a solvent solution of a ceramic oxide precursor rather than from a molten material.
This lack of understanding with respect to the formation of fine fibers from ceramic oxide precursor solution is emphasized by U.S. Pat. No. 3,485,611 to Blaze, Jr., wherein fibers are made from a fiberizable mixture such as zirconium oxide precursor dissolved in water to form a mixture having a viscosity of 40 to 45 poise. The mixture is then introduced into a hollow rotatable member having holes or openings which are sufficiently small to produce fibers having a diameter of from 2 to 3 microns. Air is used in the process merely to collect resulting fibers without attenuating the fibers. The low viscosity solutions, i.e., 40 to 45 poise, of necessity have a low solids content which results in an inferior fiber due to the requirement that large quantities of liquid be removed prior to calcining the fiber to form the oxide. Furthermore, the exceedingly small holes necessary to form the small fiber diameters readily plug thus reducing process efficiency.